You don't specify what format your Nikon is. The Canon 7D has an APS-C size sensor that results in a 1.6X magnification factor making your starting choice of 70-200 an equivalent of 112 to 320mm. In context of wedding work this would restrict him to telephoto perspective and might make overall views extremely difficult to almost impossible depending on the location.

In the '60s and '70s before zoom lenses became so popular a lot of us covered full weddings with just the "normal" lens that came with many cameras. 80mm for 120 square format, 50mm with 35mm film cameras. I photographed a "ton" of weddings with a pair of Hasselblads and just the normal 80mm F2.8 Planars they came with.

A fair answer to the OP's question might be the 18-55mm offered as a "kit" lens with the Rebel series if budget is a primary consideration or the better grade 17-55mm if he can afford it. Either of these would give him a "basic" wide angle to portrait length perspective to get started with.

Just one lens? I would recommend the EF-S 15-85mm f/3.5-5.6 IS, which provides an apparent field of view equal to 24mm at the wide end and 135mm at the telephoto end. It's probably the single best multi-purpose lens you could get for the 7D, if you're going to limit yourself to one lens.

And the answer is no, not really... the original poster says he wants just one lens, so he needs something that's wide and has some telephoto reach all at the same time. Therefore, for his particular purposes, focal length trumps maximum aperture. I don't think a fast lens is all that critical for the EOS 7D since the ISO range of that camera is so clean and high. Plus, that f/2.8 lens is $1180, making it the single most expensive EF-S lens in the line.